1. The Field of the Invention
The present invention is related to a system and process for inspecting surfaces. More particularly, the present invention is related to a system for obtaining near real time, nondestructive detection and evaluation of various materials on surfaces by directing light at the surface and analyzing the intensity and polarity of the light emanating from the surface at a wavelength corresponding to a known optical property of a predetermined material.
2. Technical Background
A typical manufacturing process utilized in many applications is the bonding of two materials. The criticality of the strength of the bond will vary depending on the particular application for which the bonded material is to be used. For example, in the manufacture of solid rocket motors, bond strength is particularly critical.
The bonds in a solid rocket motor can be subjected to forces of high magnitude due to acceleration, ignition pressurization and thermal loads. A weak bond or area of debonding can be the source of stress risers which can result in further weakening of the bond, eventually leading to failure of the bond, and can distort the geometry of the bonded material thereby adversely affecting the firing characteristics of the motor.
In the manufacture of a solid rocket motor, a variety of materials must be successfully bonded to one another. For example, some of the bonds found in a typical solid rocket motor are the bond between the case and the insulator, between the insulator and the liner, between the liner and the propellant and between the nozzle phenolic and the metal nozzle housing. A weak bond or debond in any of these bonds could result in catastrophic failure of the rocket motor.
When two materials are bonded together, contaminants on the surface of either of the materials can weaken the bond and, in some instances, cause areas of debonding. Organic materials such as greases, hydraulic fluids and mold release agents are the primary source of contamination of bonding surfaces in solid rocket motors. Other contaminants include particulates such as sand or dust. Oil vapors are often present in environments where hydraulic systems and electric motors are present. These vapors can condense on surfaces to be bonded. Even small levels of these contaminants, not visible to the human eye, can degrade bond strength.
The extent to which a surface can be cleaned prior to bonding and the method to be utilized in cleaning the surface vary according to the nature of the surface. For example, the rocket case of the space shuttle is a grit-blasted steel surface. It is typically cleaned by a vapor degrease process. According to one such process, the case is suspended within a pit in the bottom of which boiling methylchloroform is located. The methylchloroform evaporates and condenses on the rocket case. As it runs off the rocket case, it dissolves any grease in its path. While this process works well in cleaning small amounts of grease from the rocket case, if there are areas of localized buildup of grease, not all of the grease may be removed by the cleaning process.
Using a solvent such as methylchloroform to clean a bonding surface may not be viable if the bonding surface is a phenolic material. In a solid rocket motor the nozzle is typically made of a phenolic material. The nozzle is made by wrapping uncured tape onto a mandrel, permitting the tape to cure and then machining the part into the desired shape.
Phenolic materials will absorb virtually any type of cleaning solvent with which they come into contact. These solvents can alter the surface chemistry and/or carry dissolved contaminants into the phenolic. In applications such as those discussed herein, the surface properties of the phenolics must remain unchanged.
Presently, the preferred method of cleaning a phenolic material is to place it on the mill and machine a new surface, thereby removing the contaminated surface. However, this can only be done if the tolerances of the part permit a portion of the surface to be removed. Otherwise, a contaminated part may have to be replaced.
Because even small levels of contaminants, not visible to the human eye, can degrade bond strength, bonding surfaces must be inspected prior to bonding to ensure that there is no contamination, or that if there is contamination, it is within acceptable limits.
A crude method of conducting a surface inspection is to place some solvent on a wipe and stroke the surface with the wipe thereby transferring surface contaminants to the wipe. The wipe may then be analyzed using standard spectroscopy methods to verify the existence of contaminants on the wipe and determine their identity.
A principal obstacle to the successful use of this method is that it can only be used as a check method. It cannot be used as an inspection method on the entire bonding surface. And, while the method may provide information about the existence of a contaminant and its identity, it cannot be used to determine the thickness of the contamination. It is a qualitative method and therefore does not provide a quantitative measurement of the contamination. Additionally, this method cannot be used with phenolic materials because the surface chemistry of the phenolics would be altered by passing a wipe permeated with solvent over it.
A more versatile surface inspection method is to conduct a visual inspection with the aid of an ultraviolet light. Some contaminants, particularly grease such as that used for rust protection, fluoresce under ultraviolet light. Thus, by visually inspecting the surface under ultraviolet light, any contaminants which fluoresce under the light can readily be detected.
A disadvantage of this method is that the method cannot be reliably used to detect low levels of contamination as it is limited by what can be seen with the human eye. Additionally, this method, being manual in nature, does not provide machine-readable data. Consequently, the person performing the visual inspection must attempt to record the location and size of the contaminated area. As with many manual methods, the possibility of human error renders this method inadequate for many applications.
Automated inspection methods include an optically stimulated electron emission ("OSEE") method. This method is based on the photoelectric effect. By shining ultraviolet light on the surface to be inspected, electrons are emitted from the surface. By placing an electrode near the surface and raising the electrode to a predetermined voltage, an electric field is generated, drawing an electron current from the surface whose strength can be monitored. If there is contamination on the surface, the current is impeded. A disadvantage with the OSEE method is that it is subject to many variables which are not relevant to the determination of contamination. Such variables may include air currents surrounding the device being tested, relative humidity and moisture on the surface. Also, the OSEE method only works effectively on metals. It is ineffective as a tool to inspect phenolic or rubber surfaces.
Thus, it would be an advancement in the art to provide a system for the inspection of bonding surfaces which would detect the presence of thin films, including low-level contamination or surface coatings, which may not be detectible with prior-art visual inspection methods.
Indeed, it would also be an advancement in the art if such a surface inspection system could work effectively to detect contamination on a variety of surfaces and with different levels of roughness, including metal, phenolic and rubber surfaces.
It would be yet a further advancement in the art to provide such a system that could work efficiently and effectively in inspecting large surface areas.
Such a system for inspecting surfaces is disclosed and claimed herein.